The present invention is directed to coatings for high tensile steel objects, such as springs formed from high tensile steel which provide corrosion and chip-resistance to the high tensile steel.
Steel coil springs in the wheel assemblies of automobiles and other vehicles are subjected to very corrosive conditions. Conventional steel springs were expected to rust, and to allow for such rusting, conventional coil springs were formed of appropriately thick steel.
To improve the steering and ride control of automobiles, it is desirable to move the wheels outward, toward the corners of the vehicle. This increases torsional stress on the automobile body structure which must be nullified using a stronger frame assembly or weight reduction of the suspension components moved toward the corners. Reduced diameter, high tensile steel, coil springs weigh less than conventional automotive suspension coil springs, so they offer means to reduce the weight of these components. Super high tensile steel offers the promise of further weight reduction.
Herein, high tensile steel is defined as having MPa) (megapascal (N/m2) ranging from 1800 Mpa to 2100 Mpa; this includes super high tensile steel from 1950 Mpa to 2100 Mpa or above.
High tensile steel, coil springs are scratch and notch sensitive, so they require protection from impact damage caused by flying stones and gravel encountered during driving on paved or unpaved roads. Also, to maintain desired metallurgical properties and prevent premature flex damage failure, the high tensile steel cannot be heated beyond 325xc2x0 F. (163xc2x0 C.).
Conventional xe2x80x9cExe2x80x9d coat primers and/or epoxy powder coatings used on strut type automotive suspension springs are unacceptable at inhibiting flying stone damage as determined by low temperature gravelometer testing followed by accelerated scab corrosion testing of high tensile springs. Neither layer applied alone will provide the chip and corrosion protection required.
A current approach to providing corrosion and chip resistance to high tensile steel is described in U.S. patent application Ser. No. 08/728,237 filed Oct. 8, 1996, the teachings of which are incorporated herein by reference. This patent application describes a dual layer coating including a zinc-rich, thermoset, epoxy base coat which provides corrosion-resistance and a thermoplastic outer coating which provides chip-resistance. The particular thermoplastic resin taught in U.S. Pat. No. 08/728,237 is an ethylene/acrylic acid copolymer, a relatively expensive material. Polyolefins can be substituted for ethylene/acrylic acid copolymer, and although polyolefins are less expensive materials, thicker layers are required to provide the requisite chip-resistance. Also, processing of thermoplastics is expensive. A general disadvantage of such a dual layer coating is that two separate coating operations are required and two separate heating cycles are required, the first to fuse and cure, at least in part, the epoxy resin of the base coat and a second to fuse the thermoplastic resin of the outer coat plus complete curing of the epoxy base coat. Both the material of the base coat and the outer coat must be cycled at a temperature below that whereat the steel would lose its high tensile strength; accordingly, the choice of suitable resin systems for the two layers is limited.
It is accordingly an object of the present invention to provide coatings for high tensile steel which are less expensive than the coatings currently in use and which can be processed with a single heat cycle.
In accordance with the invention, chip- and corrosion-resistant coatings for high tensile steel are provided in which the resin component is entirely a xe2x80x9ctoughenedxe2x80x9d epoxy resin. Corrosion-resistance is provided by high zinc loading, particularly in the portion of the coating in contact with the high tensile steel.
As one method of toughening the epoxy resin, the epoxy is adducted to (chemically bound to) between about 5 and about 25 wt % (based on total weight of the epoxy component and the elastomer component) of an elastomer having a glass transition temperature (Tg) of xe2x88x9230xc2x0 C. or below, preferably a Tg of xe2x88x9240xc2x0 C. or below. A particularly suitable elastomeric component is carboxyl-terminated butadiene/acrylonitrile (CTBN) rubber. The elastomeric component flexibilizes the resin and remains flexible down to temperatures to which the component part may be subjected to in cold weather conditions.
In accordance with another method of toughening the epoxy resin, the epoxy resin is chemically bound as an outer shell to a soft rubber core having a Tg of about xe2x88x9230xc2x0 C. or below, preferably about xe2x88x9240xc2x0 C. or below, for example an acrylic rubber core having carboxylic acid functionality by which the epoxy resin of the shell is bound. Such acrylic rubbers might be formed from (meth)acrylic acid and (meth)acrylic esters In the fused and cured coating provided by such a powder, the acrylic rubber cores are believed to act to terminate any fracturing of the coating which may begin due to impact. In the core/shell resins which are used to form the coating powder, the acrylic rubber comprises between about 5 and about 20 wt % of the resin based on total of epoxy shell and acrylic rubber core, and the epoxy of the shell the remaining about 80 to about 95 wt %.
In accordance with another method of toughening the epoxy resin, the epoxy resin is cured with a multi-hydroxy functional curing agent having a hydroxyl equivalent weight of at least 200, preferably at least about 300, up to about 500. The distance between hydroxyl groups of such a curing agent provides flexibility to the cured epoxy resin. This toughening method may be used alone or in conjunction with either of the two above-discussed toughening methods.
Although, each of the above-described hardened epoxies exhibit chip-resistance, such chip resistance is insufficient to meet the requirements for high tensile steel springs. However, as noted above, the hardened epoxies are zinc-loaded, i.e., with at least 75 parts per hundred resin (based on total of epoxy resin, hardening resin if present, and epoxy cross-linker), preferably at least about 150 phr zinc, and more preferably at least about 200 phr zinc. Surprisingly, and unexpectedly, the zinc, added for corrosion resistance, further toughens the epoxy that is toughened by any of the three above-discussed methods. Accordingly, a toughened, zinc-loaded epoxy, at a fused and cured thickness of between about 12 and about 20 mils, preferably at least about 15 mils, may be applied as a single coat to high tensile steel, including high tensile steel springs, and meet present day commercial requirements for chip- and corrosion-resistance.
A single coat of toughened, zinc-loaded epoxy has the advantage of simplicity, being applicable in a single powder coating operation and then processed through a single heating cycle to heat and cure the epoxy resin.
On the other hand, a single coat of toughened, zinc-loaded epoxy is wasteful of zinc in that only the zinc in close proximity to the high tensile steel affords corrosion protection. As zinc is a relatively expensive material, there is provided in accordance with the invention a dual coat embodiment in which an inner coat between about 1.5 and about 3 mils thick is one of the toughened, zinc-loaded epoxies described above with respect to the single coat embodiment, and an outer coat, between about 10 and about 15 mils thick, is of the same toughened epoxy as the inner coat, but without the zinc.
As noted above, any of the above-described toughened epoxies provide chip resistance, but, insufficient chip resistance to meet current requirements for high tensile steel springs. Accordingly, the zinc-free outer coat is further toughened. One method of further toughening the outer layer is to add fibers to the outer coat formed of a material, such as glass, aramid or carbon, which does not melt or degrade at the processing temperatures to which the coating powder is subjected. Useful fibers have diameters between about 5 and about 20 xcexcm, and are used at between about 20 and about 80 phr of the resin used to form the outer coat. Another method of further toughening the outer coat is to add a blowing agent to the outer coat resin such that when processed, the density of the outer coat is reduced by at least about 25%, preferably at least 40% relative to theoretical density. Above about 65% reduction in density, the coating becomes weakened. The foaming provides porosity to the outer coating. A porous outer coat, when struck by material such as gravel may be dented, but will not fracture. Most preferably, the resin of the outer coat is further toughened by both the addition of fibers as well as by foaming through use of a blowing agent.
The dual coating is by a xe2x80x9cdry on dryxe2x80x9d method and processed in a single heating cycle. First, the toughened, zinc-loaded, epoxy coating powder is applied, e.g., electrostatically, to the high tensile steel, e.g., a spring, in an amount sufficient to form the inner coat of desired thickness. Then the coating powder for the outer coat is applied in an amount sufficient to form the outer coat of desired thickness. Then the powder coated, high tensile steel object is heated to fuse and cure the coating powders, thereby producing the dual coating. Because the same toughened epoxy resin is used to form both the inner and outer coats, the heating cycle produces a dual layer structure in which the two layers are integrally bonded to each other through the cross-linking of the epoxy resins of the inner and outer layer.
Herein, unless otherwise noted, all percentages are by weight. In the coating powders described herein, the epoxy resin and any toughening resin which becomes an integral part of the cross-linked structure are considered to be the resin system. Amounts of other materials, such as powdered zinc, glass, aramid or carbon fibers, blowing agents, etc. are expressed as parts per hundred by weight relative to the resin system calculated as 100 parts.
Herein, standards set by General Motors are considered to be a reliable, present day standard for chip and corrosion resistance, particularly General Motor""s xe2x80x9cChip Resistance of Coatingxe2x80x9d test (GM published test no. GM9508P) followed by GM""s xe2x80x9cScab Corrosion Creepback Testxe2x80x9d (GM published test no. GM9511P).
The coating powder of Zn-loaded epoxy resin may be coated directly onto a high-tensile steel component. Preferably, however, the high-tensile steel is coated first with zinc phosphate.
The epoxy resin may be chosen from a variety of epoxy resins useful for coating powders known in the art, such as those produced by the reaction of epichlorohydrin or polyglycidyl ether and an aromatic polyol such as bisphenol, e.g., bisphenol A. The epoxy resin should have an epoxy functionality greater than 1.0 and more preferably greater than 1.9. Generally the epoxy equivalent weight should be at least 170, but lower values may be possible in some cases; for example it may be 100 or more. Preferably the epoxy equivalent weight is less than 2300, especially less than 1000, for example from 130 to 1500, especially 150 to 800. Such epoxy resins may be produced, for example, by an etherificiation reaction between an aromatic or aliphatic polyol and epichlorohydrin or dichlorohydrin in the presence of an alkali such as caustic soda. The aromatic polyol may be, for example, bis(4-hydroxyphenyl)-2,2-propane (i.e. bisphenol A), bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-t-butylphenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, 4,4xe2x80x2-dihdyroxybenzophenone or 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol or dipropylene glycol, for example diglycidyl ethers or condensed glycidyl ethers of such diols, can be used. Other oxirane group-containing polymers that can be used as the epoxy resin in hybrid powder coating compositions according to this invention include polyglycidyl-functional acrylic polymers or epoxy novolak resins. Preferred epoxy resins for use in the present invention are those based on bisphenol A.
In accordance with the first epoxy-toughening method described above, the epoxy resin, preferably a bisphenol A resin, is adducted to an elastomer having a Tg of xe2x88x9230xc2x0 C. or below, preferably xe2x88x9240xc2x0 C. or below. The preferred elastomer is CTBN rubber. Such epoxy/CTBN rubber adducts are described, for example, in U.K. Patent Specification 1,407,851 (C. G. Taylor) published Sep. 24, 1975 and Powder Coatings. 184 xe2x80x9cElastomer-Modified Epoxy Powder Coatings: a Reviewxe2x80x9d, Apr. 13, 1994, No. 4347. To provide the necessary flexibilizing for cold-temperature chip resistance, the CTBN component should be present at at least about 5 wt % of the total of the CTBN and the epoxy components.
Above about 25 wt % CTBN, no further benefit is realized and it is not desired to exceed 25 wt % lest there be insufficient epoxy component for a good cure. The fact that the elastomer component is chemically bound to the epoxy component, i.e., by esterification reaction of the carboxyl groups of the CTBN with epoxy groups, ensures that a complete phase separation does not occur during fusion and curing of the coating powder. However, there are microdomains of epoxy and rubber.
In the second toughening method above, a core/shell resin is used in which an acrylic rubber resin forms the core and the epoxy resin, preferably a bisphenol A epoxy resin, forms the shell. Again, chemical bonding between carboxylic functionality of the acrylic rubber resin of the core and the epoxy resin of the shell prevents phase separation during fusion and curing of the coating powder formed using the core/shell resin. Such acrylic rubber modified epoxies are describe, for example, by Dow Chemical Company in Polymer Reprints, 32 (3) pp. 358-9 by H-J Sue and E. I. Garcia-Melfin.
Thermosetting epoxy resins contain either a cross-linking agent, such as a polyhydroxyl compound or a cure catalyst to effect auto-cross-linking of the epoxy resin.
As a third toughening method, the epoxy resin is cured with a polyhydroxyl functionality having a relatively high hydroxy equivalent weight, i.e., at least about 200 up to about 500, preferably at least about 300. The relatively high hydroxy equivalent weight of the cross-linking agent ensures relatively long chain length between OH groups, which chain lengths provide flexibility to the cured coating, helping to render the coatings chip-resistant. Suitable curing agents useful in the practice of this invention are exemplified by, but are not limited to, phenolic curing agents, such as a bisphenol A end capped diglycidyl ether of bisphenol A, which is the reaction product of a diglycidyl ether of bisphenol A and bisphenol A. Examples of preferred phenolic curing agents for the epoxy resin component includes those sold under the trademarks D.E.H.(trademark) 87 and D.E.H.(trademark) 85 by Dow Chemical Company, both of which are believed to be bisphenol A end capped diglycidyl ethers of bisphenol A. Other classes of phenolic hardeners can used as well such as phenol- and cresol-novolac curing agents sold by Georgia Pacific, Reichhold Chemicals and Ciba Geigy.
Zinc powder is added to provide corrosion resistance, but, as noted above, the Zn also acts as a toughening agent in conjunction with any of the toughened epoxies described above. Accordingly, a single coat of such Zn-rich, toughened epoxy coating can be used to form a single-coat on a high tensile steel component and provide such component with both chip-resistance and corrosion resistance. Zinc powder useful in the invention typically has an average particle size of about 4.0 microns.
Again, cost efficiencies are achieved by applying an inner coat, between about 1.5 and about 3 mils thick, of zinc-laden, toughened epoxy in contact with the high tensile steel and applying an outer coating of toughened epoxy which is free of zinc, but which either contains reinforcing fibers and/or is foamed through use of a foaming agent. Preferably, a zinc-free outer coating both contains fiber and is foamed. If a dual coat coating is formed, it is preferred that the resin composition of the two coats be substantially identical, whereby a generally continuous thermoset resin structure forms with the inner coat being rich in zinc and the outer coat being reinforced by fiber and/or foaming.
If a foaming agent is used, it may be either admixed in dry form with the coating powder, whereupon it tends to coat the surfaces of the coating powder particles, or (preferably) is integrally incorporated in the coating powder itself. If integrally mixed, the foaming agent must be activated at a temperature above that at which the coating powder composition is fused into a coating powder, but at or below the temperature at which the coating powder is fused and cured to coat the high tensile steel. Typically, foaming agent is used at between about 0.1 and about 5 phr, preferably at least about 0.5 phr, the actual amount depending upon the particular foaming agent, the particular resin system, the processing conditions and the degree to which density reduction is desired. Depending upon the amound of ingredients in addition to the resin, the foaming agent is used at between about 0.1 and about 3 wt % of the entire formulation, preferably at least about 0.3 wt %.
The foaming coating powders used in the present invention may be used by themselves to coat substrates. Generally foaming in coating powders has been considered desirable. Thus it has been a general object of coating powders to avoid out-gassing, e.g., out-gassing of water, which produce pinholes and detract from the appearance of the coating. However, in accordance with one aspect of the present invention, functional advantages are found for coating powders which produce a foamed coating. While a foamed coating is described above as a top coat in respect for a dual-coating for high tensile steel, a foamed coating may be used by itself for impact resistance where the requirements are less stringent.
Foamed coatings also provide both thermal and acoustical insulation. Thus, for example, a foamed coating might be used to coat an automotive oil pan or on the interior of automotive door panels to damp vibration.
Foamed coatings used in the present invention are not resilient, but rather indent or crush when impacted. One application for such a crushable foamed coating is in the area of providing the appearance of very minimal tolerance between component parts where manufacturing requirements allow for a greater tolerance. For example, foamed coatings on the door and door jam of a hotel safe will crush to form the appearance of an extremely tight fit even if the actual tolerance is greater.
A blowing or foaming agent to be incorporated in a coating powder must be selected so as not to foam during the heat fusion of the materials to form the coating powder, typically at a temperature of between about 180xc2x0 C. and about 240xc2x0 C. for epoxy coating powders, but produce significant gas for foaming at the fusion/curing temperature of the coating powder, typically about 300xc2x0 C. or above for epoxy coating powders. A currently preferred heat-activated foaming agent for epoxy coatings is p-toluene sulfonyl hydrazide, such as that sold as Celogen(copyright) TSH sold by Uniroyal Chemical Company. Other suitable blowing agents include, but are not limited to 2,2xe2x80x2-azobisisobutyro nitrile, dinitrosopentamethylenetetramine, sodium bicarbonate, ammonium carbonate, silicon oxyhydride and azocarbonamide.
Chemical foaming agents such as sodium borohydride might also be used to produce foaming. To foam, sodium borohydride requires a proton donor, such as water. U.S. Pat. No. 4,491,554, the teachings of which are incorporated herein by reference, describes the use in plastics of salt hydrates in conjunction with alkali metal borohydrides to produce foaming at elevated temperature, the salt hydrate only releasing water above a certain threshold temperature. For epoxy-based coating powders, the combination of sodium borohydride and alumina trihydrate is a useful combination of foaming agent and proton donor. As described in U.S. Pat. No. 4,491,554, alumina trihydrate releases no water below 260xc2x0 C. but releases significant amounts of water at temperatures above 300xc2x0 C. Thus, the coating powder can be compounded at temperatures below 260xc2x0 C. and fused/cured at temperatures above 300xc2x0 C. so as to produce a foamed coating.
The concept of foamed coating powders is not limited to epoxy-based coating powders, but applies to all types of coating powders, including, but not limited to acrylic coating powders, polyester coating powders, silicone-based coating powders, etc. The choice of foaming agents for such coating powders will be selected according to the differences in temperatures at which such coating powders are compounded and a the temperatures at which such coating powders are fused and cured.
An alternative way of producing a foamed coating in which two chemical components are required to produce gas is to prepare a dry blend of two coating powders, one of which contains one of the chemical components and the other of which contains the other chemical component. For example, an epoxy coating powder containing sodium borohydride might be dry-blended with an acrylic coating powder containing carboxylic acid functional acrylic polymers. When fused and cured, the carboxylic acid functionality of the acrylic coating powder will contribute the protons for foaming the sodium borohydride.
The coatings powders useful in the invention may also incorporate minor components known in the art, e.g., pigments, flow control agents, etc.
Coating powders used to provide the chip-resistant and corrosion-resistant coatings of the present invention are produced in the usual manner. The components are blended, and then are melt-compounded with heating above the melting point of the resin for a short time, e.g., 30-90 sec., so that no significant curing occurs. The molten compound is extruded, and after extrusion, the composition is rapidly cooled. The composition is then ground and, as necessary, the particulates sorted according to size. For electrostatic coating, the particles are generally in the 5-100 micron size range with a major portion generally being in the 20-40 micron size range. Larger particulates are useful for fluidized bed coating operations.
The invention will now be described in greater detail by way of specific examples.